Various types of oscillators are commonly used to provide a reference signal for use within electronic applications. Their piezoelectric properties allow them to be a frequency-determining element in electronic circuits. A crystal oscillator, particularly one made of quartz crystal, is distorted by an electric field when voltage is applied to an electrode near or on the crystal. This property is known as electrostriction or inverse piezoelectricity. When the field is removed, the quartz, which oscillates in a precise frequency, generates an electric field as it returns to its previous shape, and this can generate an oscillating voltage that can be used as a precise clock signal.
Typically, a crystal oscillation circuit includes a crystal oscillator, an inverter coupled in parallel with the crystal oscillator, and capacitors coupled to the input and output of the inverter and to ground. To conserve power, the crystal oscillation circuit includes an enable/disable mechanism. At certain times (e.g., when dependent electronics are in a sleep mode, or the like), the crystal oscillator can be disabled. The crystal oscillator can be started by injecting energy composed of noise and/or transient power supply response. The startup time of a crystal oscillator is typically determined by the noise or transient conditions at turn-on, small-signal envelope expansion due to negative resistance, and large-signal amplitude limiting.
It is known that crystal resistance is not constant, typically being higher at start-up than when oscillating in steady state. The crystal resistance can relate to the Q factor of the oscillator, which can dictate the amount of power applied to the crystal to keep it oscillating at a particular amplitude. As the resistance decreases, so does the amount of power consumed for maintaining oscillation at the particular amplitude. The variation in the crystal resistance causes more power to be used at start-up than is desired to achieve the best noise performance in steady state operation. However, decreasing the power such that optimal noise performance is achieved in steady state increases the amount of time for the crystal oscillator to reach steady state from start-up. Thus, there tends to be a design trade-off between power efficiency and start-up time for crystal oscillators.